The invention concerns a clamping tool for non-positive and high-precision clamping of workpieces of the type generally described in German Offenlegungsschrift 2,700,934 or German Patent Specification 2,518,382.
Clamping tasks assume a particular significance in automatic finishing, especially in metal-cutting finishing. Clamping tools whose clamping force is transferred via a pressure medium and a radially deformable expansion bush or shrinking collar onto the workpiece have proved themselves for the precise accommodation of workpieces with central bores or cylindrical outer surfaces. With these, the generation of pressure can take place through manual or machine operation of a piston inside the clamping tool, and also through the introduction of pressure by a hydraulic unit. Clamping tools of this type are distinguished by a very good long-term rotational accuracy of less than two micrometers in conjunction with high clamping force. Another possibility for the radial deformation of the expansion element consists in pressing the expansion element axially against conical surfaces. These conical surfaces can be constructed in the form of a thread, whereby the wall thickness of the expansion element is the same in the entire region in the case of a cylindrical outer surface, and whereby, moreover, ease of mounting is provided. Clamping tools with this mechanical type of deformation of the expansion element also likewise exhibit good rotational accuracy, but because of the internal friction between the body and the expansion element they often do not attain the high precision of hydraulically operated clamping elements. The type of clamping tools mentioned here can be employed to manufacture both chucks and also mandrels. No details of their construction will be given in what follows, because these clamping tools are known per se. Moreover, the clamping tools described can also be used to clamp out-of-round workpieces, or to solve other clamping tasks. A rotationally symmetric construction of the clamping element is not in any way a basic precondition for the applicability of this type of clamping tool. The clamping surfaces can also be constructed flat, to be precise when the tools likewise have flat bearing surfaces. Again, it is advantageously possible to realize permanent clamping connections in the sphere of machine elements such as bearings or shaft/hub connections with expansion elements of this type. A disadvantage of the previously described clamping tools or clamping connections consists in the only relatively slightly possible change in diameter of the expansion element. The maximum permissible change in diameter is determined by the maximum reference stress still permissible, which develops during deformation of the expansion element upon generation of the clamping force. It is customary for the expansion element to be manufactured from steel. In order to guarantee as long a service life as possible, or as high as possible a number of clampings that can be carried out by means of the clamping tool, the maximum permissible change in diameter of the expansion element may amount to only approximately three parts per thousand of the clamping diameter. A reliable clamping can therefore be attained only if the workpieces or the tools to be clamped are manufactured to a satisfactorily high quality of fit. In many instances, to meet these demands means substantial extra cost, especially in the case of small clamping diameters. Certainly, in the case of mechanically generated clamping force it is possible to realize a sizeable expansion rate by axial slotting of the expansion element, but this takes place at the expense of the rotational accuracy.
It is also known (cf. German Offenlegungsschrift 3,800,696) to manufacture the expansion elements of fibre composites in the clamping tools of the type under discussion. In this way, one certainly obtains a clamping tool which is distinguished by a permissible change in diameter of up to approximately 1%, that is to say by an expansion rate which is more than three times as high by comparison with expansion elements made of steel. However, a disadvantage of this construction is a relatively poor rotational accuracy, which is determined by the manufacture and caused by irregularities in the fibre flow during the production process; furthermore the transfer of force between the body and the expansion element presents problems. The unavoidable error in rotational accuracy amounts here to approximately five times the error in rotational accuracy attainable with steel expansion elements. In addition, sealing problems remain to be considered with this type of construction, and with hydraulic operation. The very poor wear resistance of the expansion element consisting of plastic likewise stands in the way of a wide application.
It is an object of the invention further to develop the clamping tools of the above-described types, which have metallic expansion elements and are distinguished by a high rotational or centering accuracy, high wear resistance and high clamping force, so that the requirements relating to the accuracy of fit of the bearing surfaces on the workpiece side or of the clamping surfaces on the workpiece side can be kept substantially smaller than previously.
This object is achieved according to the invention by providing an arrangement wherein the expansion element consists of an alloy with shape-memory capacity, the alloy composition being chosen so that the region of the reversible, stress-inducible austenitic/martensitic microstructural transformation, also termed pseudoelastic or superelastic region, occurs at the service temperature of the clamping tool.
According to the invention, the expansion element is manufactured from metal alloys with shape-memory capacity. Alloys of this type are known per se. The core of the invention resides in using this type of alloy in an advantageous fashion for the expansion elements of the generic clamping tools. The alloy composition for the expansion elements is chosen so that so-called pseudoelasticity occurs in the working temperature range of the clamping tool--normally room temperature. Above a critical temperature, which depends upon the composition of the memory alloy, materials with shape-memory capacity exhibit a substantially higher reversible ductility of the order of magnitude of approximately 8% by comparison with steel, with a maximum of three parts per thousand, or by comparison with other metallic alloys, with a maximum of approximately five parts per thousand. A further advantage of these alloys consists in the available basic strength and hardness of the material which it exhibits despite the enormous elasticity. Satisfactory machinability by metal cutting and satisfactory weldability of the alloy represent further advantageous properties, which first render possible a sensible use of this material to manufacture expansion elements for clamping tools operated hydraulically or mechanically. Additional corrosion protection is mostly not required, because many of these alloys are corrosion-resistant by their nature. In order to increase the resistivity with respect to abrasive wear, the expansion element can additionally be provided with a wear-resistant coating. In so doing, care must be taken to ensure that the coating adheres satisfactorily at the maximum target expansion rate of the expansion element. Candidates here are, for example, chemical nickel coatings, or microporous coatings deposited by a plasma process, or coatings deposited by a PVD or CVD process, or by a plasma nitration process, or coatings deposited by an ion implantation process. The last-named processes have the advantage that the introduction of ions into the crystal lattice produces defects which lead to compressive stresses in the region of the surface and thereby contribute to increasing the endurance strength under alternating stress.
The hardness and strength of an alloy with shape-memory capacity is comparable to steel or other usual materials. In the case of nickel-titanium alloys, apart from excellent corrosion properties and good fatigue strength, for the application according to the invention particular interest attaches furthermore to the very high reversible deformation properties with respect to the austenitic/martensitic phase transformation. An additional point is that with this type of alloy, the transformation temperature can be virtually freely adjusted between the martensitic and austenitic microstructure in the range from -100.degree. C. to +100.degree. C. through an appropriate alloy composition.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.